Indentation hardness | EPFL Graph Search

Indentation hardness tests are used in mechanical engineering to determine the hardness of a material to deformation. Several such tests exist, wherein the examined material is indented until an impression is formed; these tests can be performed on a macroscopic or microscopic scale. When testing metals, indentation hardness correlates roughly linearly with tensile strength, but it is an imperfect correlation often limited to small ranges of strength and hardness for each indentation geometry. This relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-held Rockwell hardness testers. Material hardness Different techniques are used to quantify material characteristics at smaller scales. Measuring mechanical properties for materials, for instance, of thin films, cannot be done using conventional uniaxial tensile testing. As a result, techniques testing material "hardness" by indenting a m

Damage evolution in coated cemented carbides under compressive contact loads

Maria-Simona Moldovan

This work deals principally with two important issues and their interrelation: the evolution of damage in coated cemented carbide tools used for cold forming, and the assessment of mechanical behavior of cemented carbides under compressive contact loads. Damage evolution in ironing tools is qualitatively investigated by planar and cross-sectional microscopical observations. Several mechanisms of damage are identified by taking into account the circumferential distribution along the surface of the ironing tools: coating wear and/ fracture, plastic deformation and fracture of the substrate. In the context of damage location in the substrate, the emphasis is put on the study of damage following all processing steps in substrate and coating preparation. This analysis aims at determining whether the substrate is weakened or not by the mechanical and chemical pre-treatment prior to coating. Spherical indentation testing is used as an experimental procedure to reproduce the compression stress state during cold forming. Large indenter tips (300 microns) are employed for testing cemented carbides in an effort to minimize the effects of material structural inhomogeneities. The behavior of cemented carbides under compressive loads is governed by the reversibility of deformation induced by indentation. A unique feature revealed by analysis of experimental load-displacement indentation curves is the positive energy balance, in that a gain of energy is evidenced after a loading-unloading cycle. Two hypotheses are developed for interpreting such a behavior: the strain induced martensitic transformation of the cobalt ductile phase and the thermal residual stresses in the composite. Substrate weakening subsequent to mechanical and chemical pretreatment prior to coating is expected to have a particular role on the indentation behavior of cemented carbides. The impact of substrate damage on the indentation parameters is identified by a parametric study using finite element simulations in which the damage is considered uniform. In reality, the damage distribution over the surface is rather irregular. In this context, a statistical approach is used for assessment of the material indentation parameters. The failure of coating/substrate systems under compressive contact loads is investigated by finite element simulations. The first failure events are identified by considering that system failure occurs either by substrate plastic deformation or coating fracture. The response of coated systems to spherical indentation is synthesized in damage diagrams which can predict whether the coating or the substrate fails first. The possibility for using such diagrams for assessing the evolution of damage in coated cemented carbide tools is highlighted.

EPFL2008

Solid freeform fabrication of porous ceramic parts from ceramic powders and preceramic polymers

Thomas Alexandre

This dissertation addresses the development of a Solid Freeform Fabrication process, Selective Laser Pyrolysis, to fabricate three-dimensional porous ceramic preforms from ceramic powders and preceramic polymers. It contributes to a better understanding of the physical and chemical phenomena that intervene in this process. During this study, modeling tools were adapted and characterization techniques for porous parts were developed. The first part of this work describes the fabrication of porous preforms by using Selective Laser Pyrolysis. The choice of the fabrication process is discussed, as well as the choice of the materials used. The selected materials are characterized and methods to associate them are investigated. Finally, the microstructures of the resulting porous preforms are characterized. The second part of this study is devoted to the fundamental understanding of the various phenomena occurring during the fabrication process. These phenomena are characterized or modeled. The laser-matter interaction is investigated, and a thermal model adapted to the system is proposed. This model takes into account the laser-matter interaction, the non-linear heat diffusion process and the varying properties of the materials during the process. Results of simulations are compared with experimental data that empirically validate this model. Standard characterization techniques of mechanical properties are not adapted to porous preforms fabricated in this work. Therefore, an indentation technique is developed to evaluate the effect of process parameters on the mechanical behavior. Results of simulations with the thermal model and indentation tests are used to optimize the fabrication process.

EPFL2007

Probing the buckling of pressurized spherical shells

Arefeh Abbasi, Pedro Miguel Nunes Pereira de Almeida Reis, Dong Yan

The prediction of the critical buckling conditions of shell structures is plagued by imperfection sensitivity. Non-destructive testing through point-load probing has been recently proposed to map the stability landscape of cylindrical shells. However, the counterpart procedure for spherical shells is still debatable. Here, we focus on the mechanical response of pressurized spherical shells containing a single dimple–like defect to a point probe. Combining experiments, finite element modeling, and existing results from classic shell theory, we characterize the nonlinear force-indentation response of imperfect shells at different pressurization levels. From these curves, we seek to identify the critical buckling pressure of the shell. In particular, the indentation angle is varied systematically to examine its effect on the probing efficacy. We find that the critical buckling point can be inferred non-destructively by tracking the maxima of the indentation force–displacement curves, if the probe is implemented sufficiently close to the defect. When probing further away from the defect, the test fails in predicting the onset of buckling since the deformation due to indentation remains localized in the vicinity of the probe. Using FEM simulations and shallow shell theory, we quantify the characteristic length associated with this localized deformation, both in the linear and nonlinear regimes. Our results demonstrate the limiting conditions of applicability for the usage of probing as a non-destructive technique to assess the stability of spherical shells.

2021

Damage evolution in coated cemented carbides under compressive contact loads

Maria-Simona Moldovan

EPFL2008